A screw is a type of hardware fastener that attributes its mechanical capabilities to the helical groove that extends around the circumference of the device's shank. These threads provide the friction and traction that serves a screw's purpose: to assemble or position two workpieces in relation to each other. Screws come in many varieties. The external threads on the screw can either mate with an internally threaded nut or hole, or cut its way into a soft material. Screws are inserted via torque applied--in a clockwise manner--to the head with a screw drive.

Since the terms "bolt" and "screw" were in use before the advent of easily-produced helix fasteners, they are often synonymous. However, several standards bodies have attempted to differentiate the terms, with many concluding that it is not the devices which are different, but how they are used. As provided by Machinery's Handbook and ASME B18.2.1, bolts are externally threaded fasteners that are prevented from being turned during assembly, but are positioned or released by torqueing a nut. Screws are externally threaded fasteners that can be inserted into pretapped holes or that can perforate a material and create its own internal thread. Screws are fastened by torque applied to the head. This definition is still somewhat ambiguous and is not all-encompassing, but does provide a basis to begin differentiation.

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Screw Operation

The parts on screws can be identified rather easily. The head is the section of the screw with the largest diameter and the imprint of a screw drive. This also provides a load surface once in use. The shank of the screw contains the threaded section (shoulder screws being an exception) and is responsible for the alignment of the workpieces and screw. Finally, the end of the screw that does not have the head is known as a chamfer. This tapered end allows the screw to be inserted into pieces with less resistance than the screw's full diameter. Even on machine screws, there will be a beveled edge to assist penetration.

Screws will also contain a shaped head or shaped recess so a screw driver can provide torque in the direction of the threads. While female fastener/male tool configurations are most common, male fastener/female tool variations exist. Common screw drives are pictured below:

Screw Production 

Screw Material

• Aluminum screws are light, resistant to oxidation, thermal and electrical conducive, and easy to manufacture.
• Brass screws are strong, conductive, and corrosion resistant, with low magnetic permeability.
• Copper alloy screws have good load capacity, wear resistance, and are suitable for use near magnets.
• Molybdenum screws have a very high melting point and are exceptionally strong.
• Plastic screws are inexpensive and corrosion resistant for light loads. They are common for applications near water, such as pools.
• Steel screws are produced of strong, carbonated iron. Uncoated steel is vulnerable to corrosion.
• Hardened steel screws on hardening methods to produce a stronger, but more brittle, version of steel.
• Stainless steel screws are chemical and corrosion resistant with an appealing finish. They cannot be hardened like carbon steel.
• Screws consisting of superalloys exhibit good mechanical strength, surface stability, corrosion resistance, and resistant to creep at high temperatures. Common super alloys include Hastelloy®, Inconel®, Incoloy®, and Monel®.
• Titanium screws are hard and strong, light, and corrosion resistant. When alloyed with other metals, it increases strength and durability.

Screw Finishes

Finishes applied to the base metal of the screw enhance the durability and corrosion resistance of the material.

• Black oxide finishes do not enlarge the dimensions of the screw and is a processed black rust. It is mostly used for aesthetic purposes.
• Chrome coating is a bright, reflective finish that is decorative and very durable. It is applied via electropating.
• Zinc plated coatings act as a sacrificial anode, protecting the underlying metal. It is applied as a fine white dust.
• Other coatings like galvanization and phosphating are common for particular types of hardware, like screws meant for fence or window applications.

Screw Dimensions

The chamfer angle on the underside of the head, known as the countersink, mates with a conical recess in the workpiece so the head may rest flush with the workpiece surface. ISO metric countersinks are usually 90°, while imperial screws typically have a countersink of 82°. Common countersink angles include:

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Screw Standards

Manufacturers produce screws in both imperial and metric units, and due to their explicit correspondence with nuts and pretapped holes, the units cannot be mixed. Furthermore, screws are produced with fine or coarse threads, which are a designation of the screw's thread pitch--not the quality of the product. Coarse-thread screws are less susceptible to galling, thread crossing, and seizing, while fine-thread screws are less likely to loosen from jostling, and are more easily tapped and adjusted. For screws that cut their own threads in a soft material, there is no need to determine the thread type.

Imperial

For imperial sizes, screw producers will provide a designated number or fractional inch for the dimension of the screw. North American countries follow this system, called the Unified Thread Standard. Also included will be a "threads per inch" (TPI) count which will indicate a course or fine thread.

Metric

ISO, JIS, and DIN standards are based upon the metric system and are closely related (ISO guideline 261). Most hardware measured in metric units is subject to Society of Automotive Engineers (SAE) edict as well. These standards are common in the majority of the world, and speculation exists that this may become a global hardware standard. The threads on screws can be available in both fine and course pitch configurations.

Screw Configurations

Screw Head Styles

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Screw Tip Styles

• Die points have a slight chamfer, aiding the insertion into predrilled holes.
• Dog points have an extended, unthreaded chamfer to prevent stripping fine threads.
• Rolled points have a thread roll-over on the first 540° of the helix.
• A pinch point provides a pilot action when penetrating several thin materials.
• A nail point is used to impinge or lock against wood materials.
• A cupped point increases its locking actuation under pressure.
• Round points provide gripping ability but do not 'cut' the way other screw tips do.
• A cone point is very sharp and will provide an accurate pilot for threading.
• Type-A points are best utilized as replacement screws or with predrilled holes in thin metals.
• Type-B points are similar to Type A, but are meant for thick metals.
• Type-AB combines the precision point of Type A with the ruggedness of Type B.
• Type-C points are for use in heavy sheet metal and require a high driving torque.
• Type-U points are best employed by hammering or driving.
• A type-F point has many cutting edges for tapping in large-gauge sheet metal.
• Type-FZ points have many cutting edges for tapping in plastic and other soft materials.
• Type 1 points have a groove in the chamfer for general utility.
• Type 17 points have a large groove in the shank for removing wood debris.
• Type 23 points are similar to type 17, but have a finer thread and maximize their cutting surface.
• Type 25 points are a hybrid between types 17 and 23 with a coarse thread.
• Self-drilling screw tips are self-tapping and do not require predrilling.

Types of Screws

Alike to the screw vs. bolt debate, there are often minimal differences between screw types. Denominations tend to describe the use or position of the screw as often as it describes individual screw characteristics. Below is an extensive list of screws that are readily available from both manufacturers and hardware stores.

Screw Features

Several integral features are available in screws, which increase their versatility.

• Breakaway screws have a hollowed shank that greatly reduces their shear load. This is useful in mechanical components which may experience overload, or in accident regulation. These are common in street signs and other urban furniture.

• Tamperproof screws incorporate inaccessible heads, or screw drives that are difficult to torque. This limits unauthorized removal.

Resources

Wikipedia - Screw

Mid-States Bolt and Screw